Oven for burning waste wood products

ABSTRACT

A Dutch oven for firing a boiler with waste wood products includes refractory walls with a grate structure disposed therebetween. The grate structure comprises a plurality of bars mounted upon water-cooled pipes and includes a central portion for principally supporting the fuel, and inclined side portions. A forced draft from underneath the grate is directed into the fuel as well as above the fuel through the aforementioned inclined grate side portions for providing turbulence and ovenfire burning of combustion products.

United States Patent Inventors Fred V. Sams 2981 S.W. Fairview Blvd., Portland, Oreg. 97201; Charles L. Wellons, 951 7th Ave., West Linn, Oreg. 97068; Agnes M. Sams, executrix, of said Fred V. Sams, deceased 855,691

Sept. 5, 1969 Appl. No. Filed Patented Aug. 17, 1971 OVEN FOR BURNING WASTE WOOD PRODUCTS 12 Claims, 4 Drawing Figs.

US. Cl 122/2,

, 1 10/8 R,122/375 lnt.Cl ..F22b31/00 Field of Search References Cited UNITED STATES PATENTS 912,521 2/1909 Allen 122/372 2,145,261 l/1939 Hiler 110/8 2,402,906 6/ 1946 Mosshart 122/4 X 3,027,881 4/1962 Sams 122/373 3,330,259 7/1967 Wellons 122/2 Primary Examiner- Kenneth W. Sprague Attorney-Buckhorn, Blore, Klarquist & Sparkman ABSTRACT: A Dutch oven for firing a boiler with waste wood products includes refractory walls with a grate structure disposed therebetween. The grate structure comprises a plurality of bars mounted upon water-cooled pipes and includes a central portion for principally supporting the fuel, and

I inclined side portions. A forced draft from underneath the grate is directed into the fuel as well as above the 'fuel through the aforementioned inclined grate side portions for providing turbulence and ovenfire burning of combustion products.

PATENTED M181 7 I97! SHEET 2 0F 2 FRED V. SAMS CHARLES L. WELLONS INVENTORS.

BUG/(HORN, BLORE, KLAROU/S T 8 SPAR/(MAN ATTORNEYS OVEN FOR BURNING WASTE WOOD PRODUCTS BACKGROUND OF THE INVENTION An effective means for converting waste wood, such as sawdust and scrap products produced during the manufacture of Dutchovenconventionally extends outwardly fr m u derneath the boilerflred thereby andincludes a refractory "areh for reflecting heat ontothe burning material. An extended path is provided from the body of buming fuel under rthe boiler structure wherein gaseous combustion products and the like should be finally burned. The high temperatures of the Dutch oven are. advantageous for consuming wetwaste wood products and the like. Unfortunately, even with this high heat,

. combustion is frequently incomplete resulting in low'efficiency, slow burning, and excessive production of smoke. Furthermore, the high temperatures and relatively 'low fusion point ash are apt to result in slagging andhigh maintenance costs in replacing grates and refractories.

One method of aiding the combustion of the volatile gases rapidly distilled from the fuel in a Dutch oven is to introduce :SUMMARY OF THE. INVENTION According to the present invention, an oven adapted for burning particle waste wood and the like includes refractory .walls and a grate supported between said walls. The grate is then formed'from a plurality of grate bars spaced to provide flji'passage-therebetween and mounted upon a plurality of waterecooled pipes. A central portion of the grate is substantially flat or horizontal and largely supports the waste wood fuel. Side portions of the grate, joined to the central portion,

are inclined upwardly toward refractory sidewalls of the oven, the side portions of the grate supporting the fuel to a lesser extent as a result of the inclination thereof.

A forced draft is provided from underneath the grate, and duct means or partitions are preferably included such that a different stream of air is directed underneath the central portion of the grate and underneath the grate side portions, whereby the air pressures are differentially adjustable. The forced draft beneath the central portion is directed through the-fuel while the air passing through the'side portions of the grate principally passes over the fuel for causing overflre burning of combustion products. It is found the overfire air provided'in this way is quite turbulent and aids combustion to an appreciable'degree, raising the efficiency of the oven and the consumption of gaseous combustion products as well as sub-. stantially eliniinating the production of smoke. The overall fuel burning rate is increased. In addition, since the major portion of the fuel does not reach the oven sidewalls due to the inclination of the grate side portions. the burning or scorching of the refractory sidewalls is greatly reduced. The water cooling of the grate allows the grate surface to reach high temperatures in a controlled manner for effective buming of wet fuel and yetholds heat below the point of grate destruction or slag rma ion It is accordingly an object of the present invention to provide an improved oven means adapted for burning particle waste. fuel and the like wherein the fuel-buming rate is.

increased and smok generation is decreased.

It, is another objectof the present invention to provide an improvqdoven for burning particle waste wood products. and the like wherein the ovens refractory sidewalls and grate are protected frorn burning or disintegration.

It is a; further object of the present invention to provide an improved oven for burning waste wood products and the like adapted to burn combustion, products ssociated with high temperature burning of such fuel.

lumber, into heat comprises a Dutch oven arrangement. A

' It is a further object of the present invention to provide improved apparatus for heating a boiler with waste wood fuel products.

The subject matter which we regard as our invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements.

DRAWINGS FIG. 1 is a cross-sectional view of a boiler and'furnace of the Dutch oven type;

FIG. .2 is a horizontal cross section of a Dutch oven in accordance with the present invention;

FIG. 3 is a vertical cross section of a Dutch oven in accordance with the present invention taken at 3-3 in FIG. 3; and 1 FIG. 4 is a detail of a grate support structure employed in the Dutch oven according to the present invention.

DETAILED DESCRIPTION Referring to FIG. 1, a boiler 10 including drums l1 and boiler tubes 12 is tired by a Dutch oven 14 extending outwardly from under the boiler 10. The boiler is suitably utilized in a sawmill or the like for heating dry kilns employed in processing lumber. The oven 14 includes a front wall 16, and a rear bridge wall 18 over which oxidized and partially oxidized combustion products pass for heating boiler 10 before removal through a stack (not shown). Oven walls are formed of refractory material. The unburned gaseous combustion products are to some extend consumed or oxidized in the region of the boiler, with this action being enhanced according to the present invention.

The oven 14 also includes a roof or arch 20 provided with feed ports 22 through which wood waste products such as sawdust, scrap, shavings, and the like are delivered to form piles 24 of such fuel resting upon a grate 26 supported between end walls 16 and 18. As thus far described, the construction is similar to prior art Dutch oven-type furnaces wherein, as described above, combustion has been incomplete, resulting in a low fuel burning rate, the production of excessive smoke, etc. An oven structure in accordance with the present invention is illustrated in horizontal and vertical cross section in FIGS. 2 and 3, respectively, wherein like elements are referred to employing the same reference numerals as FIG. 1.

Referring to FIGS. 2 and 3, a Dutch oven according to the present invention includes a plurality of parallel fluid-carrying pipes 28 and 58 connected in conventional fashion by return bend fittings 30 to provide a continuous passage for the circulation of a fluid such as water between inlet pipe 32 and outlet pipe 34. Water may be circulated in pipes 28 and 58 by means ofa pump (not shown and is employed for cooling grate 26. The heated water may then be used for feeding boiler 10.

The return bend fittings 30 suitably extend over a ledge 33. The pipes 28 are also supported by a central bearer support 36,, which may comprise a large pipe as illustrated in FIG. 4, and upon'level plates 38 located at each end of the Dutch oven near returnbend fittings 30. Bearer support 36 and level plates 38 are in turn'attached to the oven walls as hereinafter more fully described.

Grate 26 comprises an assemblage or bed of grate bars forming a grid as more completely described and claimed in U.S. Pat. 3,027,88l, issued to Fred V. Sams on Apr. 3, 1962. In general, the grate bars include a row of identical side bars 40 on each side of the grate, and rows of identical intermediate bars 42. Each. bar is clamped onto a pair of pipes by means of a bolt 44 extending through the bars and engaging a clamp member 46 on the opposite side of a pair of pipes. Each of the grate bars 42 comprises an elongated block having a recesses of the adjacent bar. The bars are spaced, however, to

provide winding air spaces, 50, instead of straight air passages, th'us better to hold and retain fuel on the fuel supporting top surfaces of the bars. The side faces of the bars are similarly spaced to provide air passages therebetween.

The side bars 40 are generally similar to theintermediate bars 42 except that each bar 40 has one end face 52 which is substantially normal to the long axis of the bar. and one end face 54 which is oblique thereto. This facilitates positioning of ends 52 in proximity to oven sidewalls 56 to prevent undue loss of fuel between the ends of bar 40 and such wall. End faces 54 are oblique to provide a narrow, closely spaced passage between such face 54 andla similarly oblique end face on adjacent central grate bars 42a.

As is most clearly illustrated in FIG. 3, the side bars 40 are disposed with their top surfaces defining inclined planes relative to central bars 42 and are mounted upon pipes 58 forming a part of the overall fluid pipe circuit except for being prgressively upraised adjacent sidewalls 56 so as to support the side bars 40 in their angular position. The side bars 40 extend from the ends of adjacent central bars, such as bars 42a-in FIG. 3, and angularly'upwardly toward a sidewall 56.

Pipes 28 are supported midway of the oven on an elongated fin or strut 60 secured to the top of bearer support 36- as by welding and extending vertically upward therefrom. Pipes 28 rest upon strut 60 between clamping members 46 as can be seen in FIG. 4. Strut 60 is provided with gussetlike angular end portions 62 which are cut out at 64 for holding pipes 58. Pipes 58 may be welded to the gussetlike end portions if so desired, but welding is generally not necessarily required.

surfaces of which extend angularly upward from the central portion toward the sidewalls 56. When the fuel is fed into the The above-described construction may comprise one of several combustion chambers of a multiple 'chamber'Dutch oven. arrangement as illustrated in FIGS. 2 and 3. Thus, a second chamber 66 is suitably disposed in side-by-side relation to the first, and will similarly communicate with the boiler over bridge wall 18 as has been illustrated in FIG. 1. As many chambers as desired may be arranged in s'ide-by-side relation for firing one or more boilers, thereby permitting shutdown of one chamber for cleanout or the like.

The bearer support 36 extends through sidewalls 56, for example via an aperture 68 therein, with the pipe resting upon a flat plate 70 welded underneath support 36 and resting upon the inside lower wall of aperture 68. Plate 70 prevents burning of the refractory wall by heat from support 36. A common support 36 may thus extend through as many side-by-side oven chambers as employed in one installation. Of course, the aperture 68 associated with the terminating wall of an oven structure need not extend all the, way through such wall, but only far enough for adequately supporting one end of member 36.

Adjacent the end walls 16 and 18, level plates 38 are secured to the underside of the fluid-carrying pipes. As the 10- cations of these level plates, clamping members 46 are not employed for securing grate bars to the pipes, but rather bolts 71 secure a last row of grate bars directly to the level plate 38.

The last bar 40 on each side is similarly bolted to an angularly upwardly disposed extension 72 of such level plate. Level plates 38 are suitably secured to the walls of the oven, for example a plate 38 may extend onto ledge 33.

The grate structure is conceived as comprising three portions, a central portion comprising grate bars 42 the top surfaces of which provide a substantially horizontal planar sur face, and two side portions defined by sidebars 40, the top oven via ports 22, piles 24 of such fuel rest principally upon the central horizontally flat portion of the grate. The fuel has less tendency to accumulate upon the inclined side portions of.

the grate defined by bars 40 because of their location and because of the inclination thereof. The sidewalls, formed of or lined'with refractory material, will have less tendency to burn or pit since there is less burning right at the walls. The walls are for the most'part protected, in effect, from the burning fuel by the inclined grate portions which act to direct the fuel away from the sidewalls. At most, only a thin layer of fuel is carried upon the grate side portions, e.g. at locations aligned with the center of a pile. Also, with the elongated arrangement for each of the oven chambers whereby two or more piles may be disposed in a line, relatively less of the fuel will lieproximate any oven wall and more will be located merely adjacent another pile of fuel.

The oven is also provided with means for supplying a forced draft of air from underneath the grate. Referring to FIG. 3, this means comprises a fan or blower .74. The forced draft underneath the central portion of the grate provides combustion air for the fuel while forced air from the side portions of the grate primarily flows through these angular side portions and enters the overfire region of the oven. The passage of air is'for the most part not impeded by the relatively small amount of fuel supported on the inclined side portions of the grate. In some locations the side portions will be lightly covered with fuel and at other locations not covered at all. Overfire air introduced in this manner has been found to cause considerable air turbulence around and over the burning material. The air thus introduced does not stratify, but rather mixes with the uriburned combustion products over the fire resulting in more complete burning of such products. This burning provides more heat to the boiler, a reduction in smoke, and also results in an increase in fuel burning rate as well as an increase in oven efficiency. Despite the rapid burning and attendant high heat, the water-cooled grates permit the fuel to reach a high temperature, e.g. for igniting wet fuels, while holding the grate temperature below the point of destruction or slag formation.

To provide enhanced control of the forced draft, partitions 76 and 78 are advantageously added below the grate for dividing the undergrate region into separate air ducts corresponding approximately to the central grate portion and the inclined side portions. In the present embodiment, the partitions '76 and 78 extend upwardly and adjoin selected of the pipes 28 which are inward from the inclined side portions. The exact pipes with which partitions 76 and 78 are aligned are not important so long as the partitions divide the underfire region approximately into center and side portions. Each of the partitions is cut out for the passage of pipe support 36 as can be seen at 22 in FIG. 4. The resulting three duct portions, defined by partitions 76 and 78 together with sidewalls 56, suitably be provided through the fuel bed itself. However, with the duct arrangement as illustrated in FIG. 3, the air underneath the different grate portions can be controlled by means of dampers 86, 88, and 90 so that a higher pressure, for example, can be maintained under the central portion of the grate with reduced air pressure provided under the side portions. By this means, theamount of air can be regulated tothe desired a degree. The combination of air control and water cooling pro- I vides maximum control over the speed of combustion and the temperature of combustion.

I While we have shown and described a preferred embodirnent of our invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from our invention in its broader aspects.

We claim: l. Oven means adapted for heating a boiler with particled waste wood fuel, said oven means comprising:

refractory lined side and end walls and a grate supported between said walls, said grate including a substantially planar and horizontal central portion for principally supporting said fuel during combustion, and inclined side portions extending from 'the plane of the central portion angularly upwardly toward said sidewalls, said side portions supporting said fuel to a lesser extent as a result of the inclination thereof, Said grate comprising a plurality of grate bars spaced to provide air passage therebetween, said grate bars having substantiallyfcoplanar fuel-supporting top surfaces in the central portion of said grate with the grate top bar surfaces in said side portions being coplanar within inclined side planes defined thereby, means for providing a forced draft of air from beneath said grate for passing through said grate to said fuel as well as through; said side portions to provide turbulence and overfir'e burning of combustion products, including a plurality of fluid-carrying pipes connected in a common fluid circuit for cooling said bars and upon which said bars are mounted, said pipes extending in substantially parallel array between said end walls and in a substantially common plane except for progressively upraised pipes adjacent said sidewalls which support said bars comprising said side portions.

2. Oven means adapted for heating a boiler with particled waste'w'ood fuel,- said oven means comprising:

refractory-lined side and end walls and a between said walls, said grate. including a substantially planar and horizontal central portion for principally supporting said fuel during combustion, and inclined side portions extending from the plane of the central portion angularly upwardly toward said sidewalls, said side portions supporting said fuel to a lesser extent as a result of the inclination thereof,

' said grate comprising a plurality of grate bars spaced to provide air passage therebetween, said grate bars having substantially coplanar fuel-supporting top surfaces in the central portion of said grate with the grate bar top surfaces in said side portions being coplanar within inclined side planes defined thereby, v fiteans for providing a forced draft of air from beneath said grate for passing through said grate to said fuel as well as through" said side portions to provide turbulence and overfireburning of combustion products,

air duct means underneath said grate at least approximately separating the region under the central portion of said grate from the region under said side portions, said means for providing a forced draft supplying air separately to said regions.

, 3. The oven means according to claim 2 further including means for separately adjusting the flow of air to said regions for selectively providing greater air pressure underneath said gratecentral portion.

I. Theo'ven means according to claim 1 wherein said bars have undulated side faces each defined by alternating projectionsand recesses of generally complementary configuration with the projections on one bar fitting in interdigitated relation withrespect to projections on an adjacent bar, wherein the projections on one side face of a given bar are aligned with recesses on the other side face of the same bar, the end faces of at least certain of the bars being parallel and extending obliquely with respect to the longitudinal axes of the bar.

5. Theoven means according to claim 4-wherein said bars are mounted across said pipes with long axes of said bars substantially perpendicular to the length of. said pipes, each bar grate supported 6 1 being clamped to at least two such pipes, theendfaces of bars forming a part of said grate side portions being;complementally adjacent and substantially obliquely parallel to end faces of bars comprising the central grate portion for providing a narrow winding air passage therebetween.

6. A Dutch oven for heating a boiler with particle waste wood fuel, said oven including a chamber extending from under said boiler and comprising refractory lined side and end walls with one of said end walls comprising a bridge wall for providing communication of heat to said boiler, said oven also including a heat reflecting arch provided with a plurality of fuel receiving ports centrally between said sidewalls,

a grate supported between said walls, said grate comprising a substantially planar and horizontal central portion under said fuel receiving ports for principally supporting saidsaid fuel for combustion, and inclined side portions extending from the plane of the central portion angularly upward toward said sidewalls where said fuel is accumulated to a lesser extent,

said grate including a plurality of grate bars narrowly spaced to provide narrow air passages and a plurality of fluid carrying pipes connected in a common fluid circuit for cooling said bars and upon which said bars are supported, said pipes extending in substantially parallel array between said end walls and in a substantially common plane except for progressively upraised pipes adjacent said sidewalls and supporting said bars which comprise said side portions, the bars comprising the central and side portions being closely spaced at the side margins of said central portion, 1

means for providing a forced draft of air from beneath said grate passing through said grate to said fuel and providing appreciable air flow through said side portions for causing turbulence and overfire burning of combustion products, and partition means for at least approximately dividing said forced draft as applied underneath said central and side portions.

7. The oven according to claim 6 wherein said bars have undulated side faces each defined by alternating projections and recesses of generally complementary configuration with the projections on one bar fitting in interdigitated relation with respect to projections on an adjacent bar wherein the projections on one side face of a given bar are aligned with recesses on the other side face of the same bar, the end faces of at least certain of the bars being parallel and extending obliquely with respect to the longitudinal axes of the bar, said bars extending with their long axes substantially perpendicular to the length of said pipes on which said bars are supported.

8.'The oven according to claim 6 wherein said means for providing a forced draft comprises a blower, said oven further including plural ducts leading from said blower to the undergrate regions separated by said partitions, and damper means in said ducts for separately adjusting the air provided underneath said central portion from the air provided underneath the side portions of said grate.

9. The oven according to claim 6 further including central support means underneath said grate extending at least partially through apertures provided in said sidewalls, said support means having an elongated upwardly extending strut upon which said pipes rest.

10. The oven according to claim 6 wherein said oven chamber is elongated outwardly from under said boiler for accommodating at least a pair of fuel piles located under corresponding fuel receiving ports with one such pile being disposed outwardly from the other,

said oven including further similar chambers having side and end walls with similar grates mounted therebetween on a plurality of of fluid-carrying pipes, said further chambers being disposed in side-by-side relation with the first mentioned chamber and communicating with the region of said boiler, said means for providing a forced draft being common to said chambers.

l L The oven according to claim 10 further including a com- 12; The oven"according to claim 1 1 further including a plate mon support means extending through sidewalls in adjacent disposed in sidewall apertures through which said support oven chambers for'centrally supporting the grates of plural 7 means extends and upon which said support means rests. oven chambers. a

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 599 609 Dated August 17 197].

Inventor s Fred V. Sams and Charles L. Wellons It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

line 36 "extend should be -extent;

Column 2, I

Column 3, line 8, first "are" should be as-; Column 4, line 24, after "from" insert -under;

Column 5, claim 1, line 19, "grate top bar" should be grate bar top; Column 5, claim 1, line 27, before including" insert -said grate-.

Signed and sealed this 22nd day of February 1972.

(SEAL) Attest:

EDWARD M.FLETC HER, JR. B T TTACHALK Attesting Officer Commissioner of Patents JRM PC1-1050 {10-59) USCOMM-DC 60376-P69 0 U S GOVERNMENY FRINYING OFFH'I I959 0* 366*334 

1. Oven means adapted for heating a boiler with particled waste wood fuel, said oven means comprising: refractory lined side and end walls and a grate supported between said walls, said grate including a substantially planar and horizontal central portion for principally supporting said fuel during combustion, and inclined side portions extending from the plane of the central portion angularly upwardly toward said sidewalls, said side portions supporting said fuel to a lesser extent as a result of the inclination thereof, Said grate comprising a plurality of grate bars spaced to provide air passage therebetween, said grate bars having substantially coplanar fuel-supporting top surfaces in the central portion of said grate with the grate top bar surfaces in said side portions being coplanar within inclined side planes defined thereby, means for providing a forced draft of air from beneath said grate for passing through said grate to said fuel as well as through said side portions to provide turbulence and overfire burning of combustion products, including a plurality of fluid-carrying pipes connected in a common fluid circuit for cooling said bars and upon which said bars are mounted, said pipes extending in substantially parallel array between said end walls and in a substantially common plane except for progressively upraised pipes adjacent said sidewalls which support said bars comprising said side portions.
 2. Oven means adapted for heating a boiler with particled waste wood fuel, said oven means comprising: refractory-lined side and end walls and a grate supported between said walls, said grate including a substantially planar and horizontal central portion for principally supporting said fuel during combustion, and inclined side portions extending from the plane of the central portion angularly upwardly toward said sidewalls, said side portions supporting said fuel to a lesser extent as a result Of the inclination thereof, said grate comprising a plurality of grate bars spaced to provide air passage therebetween, said grate bars having substantially coplanar fuel-supporting top surfaces in the central portion of said grate with the grate bar top surfaces in said side portions being coplanar within inclined side planes defined thereby, means for providing a forced draft of air from beneath said grate for passing through said grate to said fuel as well as through said side portions to provide turbulence and overfire burning of combustion products, air duct means underneath said grate at least approximately separating the region under the central portion of said grate from the region under said side portions, said means for providing a forced draft supplying air separately to said regions.
 3. The oven means according to claim 2 further including means for separately adjusting the flow of air to said regions for selectively providing greater air pressure underneath said grate central portion.
 4. The oven means according to claim 1 wherein said bars have undulated side faces each defined by alternating projections and recesses of generally complementary configuration with the projections on one bar fitting in interdigitated relation with respect to projections on an adjacent bar, wherein the projections on one side face of a given bar are aligned with recesses on the other side face of the same bar, the end faces of at least certain of the bars being parallel and extending obliquely with respect to the longitudinal axes of the bar.
 5. The oven means according to claim 4 wherein said bars are mounted across said pipes with long axes of said bars substantially perpendicular to the length of said pipes, each bar being clamped to at least two such pipes, the end faces of bars forming a part of said grate side portions being complementally adjacent and substantially obliquely parallel to end faces of bars comprising the central grate portion for providing a narrow winding air passage therebetween.
 6. A Dutch oven for heating a boiler with particle waste wood fuel, said oven including a chamber extending from under said boiler and comprising refractory lined side and end walls with one of said end walls comprising a bridge wall for providing communication of heat to said boiler, said oven also including a heat reflecting arch provided with a plurality of fuel receiving ports centrally between said sidewalls, a grate supported between said walls, said grate comprising a substantially planar and horizontal central portion under said fuel receiving ports for principally supporting said said fuel for combustion, and inclined side portions extending from the plane of the central portion angularly upward toward said sidewalls where said fuel is accumulated to a lesser extent, said grate including a plurality of grate bars narrowly spaced to provide narrow air passages and a plurality of fluid carrying pipes connected in a common fluid circuit for cooling said bars and upon which said bars are supported, said pipes extending in substantially parallel array between said end walls and in a substantially common plane except for progressively upraised pipes adjacent said sidewalls and supporting said bars which comprise said side portions, the bars comprising the central and side portions being closely spaced at the side margins of said central portion, means for providing a forced draft of air from beneath said grate passing through said grate to said fuel and providing appreciable air flow through said side portions for causing turbulence and overfire burning of combustion products, and partition means for at least approximately dividing said forced draft as applied underneath said central and side portions.
 7. The oven according to claim 6 wherein said bars have undulated side faces each defined by alternating projections and recesses of generally complementary configuration with the projections on one bar fitting in inteRdigitated relation with respect to projections on an adjacent bar wherein the projections on one side face of a given bar are aligned with recesses on the other side face of the same bar, the end faces of at least certain of the bars being parallel and extending obliquely with respect to the longitudinal axes of the bar, said bars extending with their long axes substantially perpendicular to the length of said pipes on which said bars are supported.
 8. The oven according to claim 6 wherein said means for providing a forced draft comprises a blower, said oven further including plural ducts leading from said blower to the undergrate regions separated by said partitions, and damper means in said ducts for separately adjusting the air provided underneath said central portion from the air provided underneath the side portions of said grate.
 9. The oven according to claim 6 further including central support means underneath said grate extending at least partially through apertures provided in said sidewalls, said support means having an elongated upwardly extending strut upon which said pipes rest.
 10. The oven according to claim 6 wherein said oven chamber is elongated outwardly from under said boiler for accommodating at least a pair of fuel piles located under corresponding fuel receiving ports with one such pile being disposed outwardly from the other, said oven including further similar chambers having side and end walls with similar grates mounted therebetween on a plurality of of fluid-carrying pipes, said further chambers being disposed in side-by-side relation with the first mentioned chamber and communicating with the region of said boiler, said means for providing a forced draft being common to said chambers.
 11. The oven according to claim 10 further including a common support means extending through sidewalls in adjacent oven chambers for centrally supporting the grates of plural oven chambers.
 12. The oven according to claim 11 further including a plate disposed in sidewall apertures through which said support means extends and upon which said support means rests. 